unit V rectifiers and power supplies

rectifiers and power supplies derivation and problems

1. UNIT V POWER SUPPLIES AND ELECTRONIC DEVICE TESTING

2. Rectifiers

3. Block diagram of Power Supply

4. Rectifier A circuit that converts ac voltage of main supply into pulsating dc voltage using one or more pn junction diodes. Half Wave Rectifier Full Wave Rectifier Center Tap Rectifier Bridge Rectifier

5. Important Characteristics of Rectifier Waveform of the load current Regulation of output voltage Rectifier efficiency Peak value of current in the rectifier circuit Peak value of voltage across the rectifier element in the reverse direction ( PIV) Ripple factor

6. Half Wave Rectifier Circuit Diagra m

7. Operation of Half Wave Rectifier

8. Waveform of Half Wave Rectifier

9. Average DC load Current (I DC )

10. Average DC voltage ( Edc )

11. RMS Load Current ( Irms ) RMS Load Voltage ( Erms )

12. DC Power Delivered to the load

13. AC input power from transformer secondary

14. How effectively a rectifier converts ac into dc: Rectifier Efficiency ( η ) Ripple Factor (r)

15. Rectifier Efficiency ( η ) Tells us the percentage of total input ac power that is converted into useful dc output power. η = 40.6 % Under best conditions (no diode loss) only 40.6% of the ac input power is converted into dc power . The rest remains as the ac power in the load

16. Ripple Factor Measure of purity of the dc output of a rectifier Defined as the ratio of ac component of the output wave to the dc component in the wave

17. Ripple Factor This indicates that the ripple content in the output are 1.211 times the dc component. i.e. 121.1 % of dc component. The ripple factor is very high. Therefore a half wave rectifier is a poor converter of ac to dc. The ripple factor is minimized using filter circuits along with the rectifier.

18. Peak Inverse Voltage (PIV) PIV = Em Diode must be selected based on the PIV rating and the circuit specification.

19. Disadvantage of HWR The ripple factor of half wave rectifier is 1.21, which is quite high. The output contains lot of ripples The maximum theoretical efficiency is 40%. The practical value will be quite less than this. This indicates that HWR is quite inefficient.

20. Full WaveRectifier

21. Working of Center Tap Rectifier Current Flow during the positive half of the input cycle Current Flow during the negative half of the input cycle

22. Waveforms

23. Average DC current Average (DC) Voltage

24. RMS Load Current ( Irms ) RMS Load Voltage

25. DC Output Power AC input power (Pac)

26. Rectifier Efficiency ( η )

27. Ripple Factor This indicates that the ripple contents in the output are 48% of the dc component which is much less than that for the half wave rectifier.

28. Peak Inverse Voltage

29. Advantages of Full Wave Rectifier Efficiency is higher. The large dc power output The ripple factor is less Disadvantages of Full Wave Rectifier PIV rating of diode is higher. Higher PIV diodes are larger in size and costlier. The cost of center tap transformer is high.

30. Bridge Rectifier

31. Working of Bridge Rectifier

32. Waveforms of Bridge Rectifier

33. Parameters :

34. Advantages of Bridge Rectifier It does not need center tap transformer secondary. The transformer secondary voltage of CT rectifier is 2Vm, where as in Bridge the transformer secondary must have a peak voltage of Vm . That is the transformer secondary of CT rectifier must have double the number of turns. Such transformers are costlier. If stepping up or stepping down of voltage is not needed, we may even do away without transformer. Each diode in center tap has a PIV rating of 2Vm, whereas diodes in bridge rectifier needs a PIV rating of Vm . Hence the diodes for use in center tap rectifier are costlier than meant for bridge rectifier.

35. Disadvantages of Bridge Rectifier It requires four diodes, two of which conduct in alternate half cycles. This creates a total voltage drop of 1.4V (if Si diodes are used). Therefore this creates a problem if low dc voltage is required. The secondary voltage is low and two diode voltage drop of 1.4V becomes significant.

36. Rectifiers with filter Pulsating DC not constant (fluctuates w.r.t time) This is not applicable to electronic components Solution – smoothing the fluctuating DC (with filter) It consists of A transformer Diode Capacitor

37. Operation Positive half cycle – diode FB – capacitor charges Negative half cycle – diode RB – capacitor discharges

38. Output graph Presence of capacitor – comparatively smooth DC

39. Full wave rectifier with filter To improve conduction cycle It consists of Centre tapped transformer 2 diodes Capacitor

40. Operation and output Positive half cycle – 2 diodes FB – capacitor charges Negative half cycle – 2 diodes RB – capacitor discharges

41. Voltage Regulators Delivers constant power to the load V o controlled by internal circuitry Avoids unnecessary change in load Figure shows role of a regulator

42. Continued.. Basic diagram is as follows Has 3 parts Reference voltage circuit Feedback circuit (error amplifier) Current amplifier Figure of merit for voltage regulator Line regulation Load regulation

43. Output resistance Relates output voltage and current Ideal case – V o independent of I o Practical case - V o is a function of I o This relation is R o R o = change in V o / change in I o

44. Types of voltage regulator Series regulator Shunt regulator based on configuration between transistor (with load) and control element Switching regulator

45. Series regulator Control element is in series with the load Basic block diagram is as follows

46. Continued.. Series regulator consists of the following C ontrol element Reference voltage Sampling circuit Comparator circuit Input is unregulated power supply Functions of each block

47. Continued.. Circuit diagram of series regulator is It consists of the following S ource resistance R Transistor Q1 Zener diode Load

48. Continued.. I o = V o / R L (load current)

49. Shunt regulator Shunt configuration between control element and the load Shunts current from the load The block diagram is as follows

50. Continued.. Circuit diagram of series regulator is It consists of the following Source resistance R Transistor Q1 Zener diode Load The Z ener diode, Transistor and Load are parallel to each other

51. Switching regulator Completely different from series and shunt More efficient power transfer to load Added circuit complexity It consists of a V-source, Transistor, Pulse generator and Filter

52. Continued.. The basic circuit diagram is as follows It consists of Voltage reference - Diodes Error amplifier - R and L components Pulse wave generator

53. Types of switching regulators Step down switching regulator Step up switching regulator Inverting type switching regulator

54. Step down switching regulator It consists of a transistor (Q), inductor, capacitor, comparator, oscillator, zener diode Reference voltage – zener diode Oscillator controls Q for switching L and C acts as filter

55. Step up switching regulator Basic elements identical to step down type

56. Inverting type switching regulator Identical elements to previous types But connections different Output - opposite polarity to input

57. Continued.. On/Off period of Q controlled by pulse width oscillator Q – saturation , Diode is RB

58. Over voltage protection Switching regulator used (step down) Excessive input voltage identified by peak current of regulator External transistor biased using R B